This invention relates to a vibratory transducer which is particularly suited for use in a Coriolis mass flowmeter.
To determine the mass flow rate of a fluid flowing in a pipe and particularly of a liquid, use is frequently made of measuring devices which induce Coriolis forces in the fluid and derive therefrom a measurement signal representative of mass flow rate by means of a vibratory transducer and control and evaluation electronics connected thereto.
Such transducers and particularly their use in Coriolis mass flow meters have been known and in industrial use for a long time. U.S. Pat. No. 5,549,009, for example, discloses a Coriolis mass flowmeter incorporating a vibratory transducer which responds to the mass flow rate of a fluid flowing in a pipe and comprises:
a curved flow tube for conducting the fluid which vibrates in operation and communicates with the pipe via an inlet-side tube section and an outlet-side tube section.
an antivibrator which extends essentially parallel to and oscillates in a phase opposition to the flow tube and is mechanically connected with the flow tube
by means of at least a first coupler on the inlet side and
by means of at least a second coupler on the outlet side;
an excitation system for driving the flow tube and the antivibrator at an excitation frequency; and
a sensor system for sensing inlet-side and outlet-side vibrations of the flow tube,
wherein a torsional rigidity of the inlet-side tube section and a torsional rigidity of the outlet-side tube section are adapted to one another and to an internal system supported by the two tube sections and formed by at least the flow tube, the antivibrator, the excitation system, and the sensor system such that the internal system is suspended essentially xe2x80x9ctorsionally softxe2x80x9d, i.e., in a torsionally nonrigid manner.
As is well known, vibrating flow tubes, for example U-, V-, or xcexa9-shaped tubes, if excited into cantilever vibrations in a first natural mode, can cause Coriolis forces in the fluid passing therethrough. In such transducers, the first natural vibration mode chosen for the flow tube is usually the mode in which the flow tube oscillates about a longitudinal axis of the transducer at a lowest natural resonance frequency.
The Coriolis forces thus produced in the fluid result in cantilever vibrations of an at least second natural mode being superimposed on the excited, pendulum-like cantilever vibrations of the so-called useful mode, the vibrations of the second mode being equal in frequency to those of the useful mode. In transducers of the kind described, these cantilever vibrations forced by Coriolis forces, the so-called Coriolis mode, commonly correspond to the natural mode in which the flow tube also performs torsional vibrations about a vertical axis that is perpendicular to the longitudinal axis. As a result of the superposition of the useful and Coriolis modes, the flow tube vibrations sensed on the inlet and outlet sides of the tube by means of the sensor system have a measurable phase difference, which is also dependent on mass flow rate.
Frequently, the flow tubes of such transducers, which are used in Coriolis mass flowmeters, for example, are excited in operation at an instantaneous resonance frequency of the first natural mode, particularly with the vibration amplitude maintained constant. As this resonance frequency is also dependent on the instantaneous density of the fluid in particular, commercially available Coriolis mass flowmeters can also be used to measure the density of moving fluids.
One advantage of a curved tube shape is that thermally induced expansion, particularly in flow tubes with a high expansion coefficient, produce virtually no or only very slight mechanical stresses in the flow tube itself and/or in the connected pipe. Another advantage of curved flow tubes is that the flow tube can be made relatively long, so that high sensitivity of the transducer to the mass flow rate to be measured can be achieved with a relatively short mounting length and relatively low excitation energy. These circumstances permit the flow tube to be made from materials having a high expansion coefficient and/or a high modulus of elasticity, such as special steel.
In vibratory transducers with a straight flow tube, the latter is commonly made from a material having at least a lower expansion coefficient and possibly a lower modulus of elasticity than special steel in order to avoid axial stresses and achieve sufficient measuring sensitivity. Therefore, such straight flow tubes are preferably made of titanium or zirconium, but because of the higher material cost and the generally higher machining cost, such tubes are far more expensive than those made of special steel.
Transducers of the kind disclosed in U.S. Pat. No. 5,549,009, i.e., transducers with a single curved flow tube and with an antivibrator, particularly one extending parallel to the flow tube, have proved especially effective in applications where the fluid to be measured has an essentially constant or only very slightly varying density. For such applications, it is readily possible by means of the antivibrator oscillating in operation at the same frequency as, but in phase opposition to, the flow tube to nearly completely neutralize those transverse forces which were induced in the transducer as a result of alternating lateral motions of the oscillating flow tube, thus virtually keeping such transverse forces away from the connected pipe.
If used for fluids with widely varying densities, such a transducer has practically the same disadvantage as a transducer without an antivibrator, particularly as compared to transducers with two parallel flow tubes.
It turned out that the aforementioned forces produced in the transducer cannot be completely balanced with such an antivibrator. As a result, the above-mentioned internal system, oscillating as a whole about the transducer""s longitudinal axis, may also start to vibrate laterally. Accordingly, these lateral vibrations of the internal system force an additional elastic deformation of the inlet-side and outlet-side tube sections and consequently may cause flexural vibrations in the connected pipe. In addition, such lateral vibrations may cause cantilever vibrations very similar to, and thus practically indistinguishable from, the Coriolis mode to be excited in the empty flow tube, and this, in turn, would render the measurement signal that ought to represent the mass flow rate of the fluid unusable.
On the other hand, as is well known, a significant advantage of a single flow tube transducer over a transducer having two parallel flow tubes is that no manifolds are necessary to connect the flow tubes with the pipe. Such manifolds, on the one hand, are expensive to make and, on the other hand, represent flow bodies with a strong tendency to sedimentation or clogging.
One way of reducing density-dependent transverse forces is described, for example, in U.S. Pat. No. 5,287,754 or in U.S. Pat. No. 5,705,754. In the transducers disclosed therein, the transverse forces produced by the vibrating single flow tube, which oscillate at medium or high frequencies, are kept away from the pipe by means of an antivibrator that is heavy compared to the flow tube, and by coupling the flow tube to the pipe relatively loosely, i.e., practically by means of a mechanical low-pass filter. Unfortunately, however, this causes the antivibrator mass required to achieve sufficient damping of the transverse forces to increase disproportionately with the nominal diameter of the flow tube.
This represents a big disadvantage for such transducers, since the use of such massive components always entails both increased assembly costs during manufacture and increased costs during installation of the measuring device in the pipe. In addition, it is difficult to ensure that the lowest natural frequency of the transducer, which decreases with increasing mass, is still far from the likewise rather low natural frequencies of the connected pipe. Thus, use of such a transducer in industrial Coriolis mass flowmeters is limited to relatively small nominal flow tube diameters up to about 10 mm.
It is therefore an object of the invention to provide a transducer which is particularly suited for a Coriolis mass flowmeter and which in operation, even if it uses only a single straight flow tube, is well balanced dynamically over a wide fluid density range and nevertheless has comparatively little mass.
To attain this object, the invention provides a vibratory transducer for a fluid flowing in a pipe, which comprises:
a curved flow tube for conducting the fluid which vibrates in operation and communicates with the pipe via an inlet-side tube section and an outlet-side tube section;
an antivibrator which in operation oscillates in a phase opposition to the flow tube and is mechanically connected with the flow tube
by means of a first coupler on the inlet side and
by means of a second coupler on the outlet side;
an excitation system for vibrating the flow tube and the antivibrator at an excitation frequency; and
a sensor system for sensing inlet-side and outlet-side vibrations of the flow tube,
wherein an internal system formed by at least the flow tube, the antivibrator, the excitation system, and the sensor system, oscillating about a longitudinal axis of the transducer which is essentially in alignment with the inlet-side tube sections, forces
a torsion of the first and second couplers about the longitudinal axis and
an essentially torsional elastic deformation of at least parts of the inlet-side and outlet-side tube sections, and
wherein in order to achieve a torsionally soft oscillation of the internal system,
at least the first coupler, tuned to a torsional rigidity of the inlet-side tube section, and
at least the second coupler, tuned to a torsional rigidity of the outlet-side tube section, are so dimensioned that
an inlet-side inherent torsion eigenmode of the first coupler and of the inlet-side tube section has a natural frequency approximately equal to the excitation frequency, and
an outlet-side inherent torsion eigenmode of the second coupler and of the outlet-side tube section has a natural frequency essentially equal to the natural frequency of the inlet-side inherent torsion eigenmode.
In a first preferred embodiment of the invention, the natural frequency of the inlet-side inherent torsion eigenmode is lower than the excitation frequency.
In a second preferred embodiment of the invention, the antivibrator extends essentially parallel to the flow tube.
In a third preferred embodiment of the invention, the antivibrator has a mass distribution at least similar to that of the flow tube.
In a fourth preferred embodiment of the invention, the antivibrator is tubular in form.
In a fifth preferred embodiment of the invention, the antivibrator is essentially coaxial with the flow tube.
In a sixth preferred embodiment of the invention, the antivibrator is essentially identical in shape to the flow tube.
In a seventh preferred embodiment of the invention, counterbalance bodies are attached to the antivibrator for adjusting the mass distribution of the antivibrator.
In an eighth preferred embodiment of the invention, the antivibrator is heavier than the flow tube.
In a ninth preferred embodiment of the invention, a first rotating-mass counterbalance body is rigidly fixed to the inlet-side tube section to adjust the inlet-side torsion eigenmode, and a second rotating-mass counterbalance body is rigidly fixed to the outlet-side tube section to adjust the outlet-side torsion eigenmode.
In a tenth preferred embodiment of the invention, the transducer comprises a first torsion absorber, which is essentially coaxial with the inlet-side tube section, and a second torsion absorber, which is essentially coaxial with the outlet-side tube section.
In an eleventh preferred embodiment of the invention, the transducer comprises a transducer case fixed to the inlet-side tube section and to the outlet-side tube section and having a lowest natural frequency which is at least 20% above the excitation frequency.
The invention further provides a Coriolis mass flowmeter incorporating a transducer as mentioned above.
A fundamental idea of the invention is to use such torsional vibrations of the internal system suspended in the aforementioned manner, which are rather uncritical for the Coriolis mode and, consequently, for the mass flow rate measurement, and which are caused essentially by in-phase oscillating motions of the flow tube and the multivibrator, to neutralize lateral transverse forces, which are extremely detrimental to the development of the Coriolis mode and, consequently, to the measurement of the mass flow rate. This means that these hitherto undesired torsional vibrations are not merely not damped out but, by being selectively adjusted for their mechanical coupling to the pipe, produced in such a way as to achieve a reduction of interfering effects on the transducer and, thus, an improvement in the transducer""s measuring properties.
To accomplish this, according to the invention, an inlet-side torsion vibrator, formed by the inlet-side coupler and the inlet-side tube section, and an outlet-side torsion vibrator, formed by the outlet-side coupler and the outlet-side tube section, are mechanically tuned so as to oscillate practically at resonance with the oscillating internal system. This serves to oscillate the internal system free from external reaction torques if possible, so that the internal system is practically perfectly isolated from the inlet-side and outlet-side tube sections. As a result of this isolation, a total angular momentum of the internal system is practically zero. To the same degree as the total angular momentum, a total lateral momentum of the internal system, and thus transverse forces derived therefrom and transmissible to the outside, are reduced to zero by this isolation.
One advantage of the invention is that the transducer is very well balanced with a relatively small additional amount of mechanical complexity, particularly over a wide fluid density range, regardless of operational variations of an internal total mass.
The transducer according to the invention is further characterized by the fact that the inlet-side and outlet-side tube sections can be kept short, so that the mounting length of the transducer can be substantially reduced while the high quality of the dynamic vibration isolation remains essentially unchanged. Despite its short mounting length, the transducer can be made very light.